Variable resistor

ABSTRACT

A variable resistor having a number of windings consisting of flat resistance strips which have been formed by etching or galvanic deposition is disclosed. The resistor has a thermally conductive base and an electrically insulating layer between the resistance strip and the base.

United States Patent 1191 [111 3,876,967 Hehl et a1. Apr. 8, 1975 [54]VARIABLE RESISTOR 2.218.204 10/1940 Marstcn 338/159 X 2. 5 ,3 5 2 1Klaus Friedrich Hehk Bemd 2.398.833 7/135? iii/16? Schenk, both ofNorderstedh 2.849.350 8/1958 338/16 x Germany 2.873.336 2/1959 Tassara338/127 3.2 1,722 9 33 AS99139 Tool Works Chmgo- 3.431.133 1/132332312331, 8 a1. 333/133? 3.544.946 12/1970 Murakami 338/174 22 i Oct 311973 3699.493 10/1972 Oka et 211.

3,748,626 7/1973 Maurice 338/184 X 21 Appl. No.: 411,580

OTHER PUBL1CAT1ONS Dummea Variable Resistors Radio and Electronic 30 F At P t I orelgn pphca Da a Components, V01. 11, Pitman & Sons. pp. 79-81.

Nov. 4. 1972 Germany 2254085 52 us c1. 338/126; 338/127; 338/159;Reyno'ds 338/161; 338/174; 338/202 [51] Int. Cl. H0lc 9/02 [57] ABSTRACT1581 gg of 8 532 A variable resistor having a number of windings cong 1g sisting of flat resistance strips which have been formed 9O by etchingorgalvanic deposition is disclosed. The resister has a thermallyconductive base and an electri- References Cited fliillyl/xilgznlatinglayer between the resistance strip and UNITED STATES PATENTS 2.213.0788/1940 Stoekle 338/159 Clams 29 Draw Fgures 3,876,967 v SHEETBUF? Fig.26

IHIIIIIIIIII4 VARIABLE RESISTOR The invention relates to a variableresistor. especially a series resistor for a D.C. series motor,comprising a resistor unit possessing a number of windings, a sliderhaving a contact which can be brought into contact with surfaces of theindividual windings. and an actuating device for the slider.

Resistors of this type, as for example are used as series resistors forthe fan motors of automobiles, are known in many forms of construction.The usual form consists of a wire resistance wound in or on a ceramicbody and provided with special fixing elements and connections. Theresistance wire is usually of iron or constantan. Resistance wires arealso known which are fitted to relatively large ceramic plates. butwhich have a relatively low specific rating. The ceramic plates normallyserve as heat removal plates. In other resistors of this type having ahigh specific rating, the resistors are often mounted outside thevehicle cabin of an automobile, to facilitate the dispersal of thethermal energy. If the above described resistors are used as seriesresistors, for example for fan motors, they are normally variable insteps or infinitely variable. This introduces certain problems,especially with regard to the form of the contact points, which arepartly quite highly loaded. For this reason, the adjusting part and theload bearing part are frequently separated. It is also a requirement insuch series resistors that they shall be as small as possible and as faras possible shall have very low surface temperatures. Their cost mustalso be as low as possible.

Resistors of so-called sandwich construction have also been disclosed,in which a light metal baseplate is laminated with glass fabricimpregnated with epoxy resin. A sheet of relatively thin constantan orof other copper nickel alloys is applied onto the glass fabric.Individual resistance strips are then formed from the sheet by galvanicremoval.

For many applications, the above described resistors can only beconsidered as partially satisfactory. The type initially described,which are constructed of ceramic bodies and resistance wire, are usuallyexpensive to manufacture. The installation and repair of these resistorsalso present problems, since riveting or fixing of the ceramic body isdifficult, due to its usually low duetility. To keep their size small,relatively thin wires must also be used. This in turn results inrelatively high surface temperatures, for example 250C and more. lfironresistance wires are used, the life of the resistors is adverselyinfluenced by susceptibility to corrosion. Resistance wires made fromcopper nickel alloys, on the other hand. have a low specific resistance,so that their size is relatively large. Round resistance wire makescontacting difficult. Furthermore, numerous small components arenecessary to make such resistors ready for installation. Due to the highsurface temperatures, actuators of thermoplastics frequently cannot beused, or special thermal insulators must be provided, in order to reducethe temperature effect upon thermoplastics actuating elements.

Resistors made from the sandwich material referred to above possess thedisadvantage that the method of manufacturing them is relativelyexpensive. Since the light metal baseplate serves as heat removal plate,it

must be of a certain size, with the consequence that quite large areasoften have to be introduced into the etching machines. Since the cost ofetching is a function of the surface etched such resistors. of a size tosuit the necessary heat removal plate, are relatively expensive. Also,the temperature distribution in such builtup resistors is oftenunfavourable and local high temperature peaks occur. leading to a riskthat the epoxy resins used as adhesive may reach the self-ignitiontempera ture range. This is an extreme disadvantage especially when aresistor of this type is used in an automobile, where burning can resultin the ignition of other components. A further disadvantage lies in thecopper nickel alloy used, which because of its good conductivity isapplied as a thin rolled sheet. A sheet of this type possesses firstlyinsufficient mechanical strength and is moreover extremely sensitive tocracking, tearing and other damage. Finally, the dielectric strength ofglass fibre mats impregnated with epoxy resin is relatively low, so thatthere is a risk of short circuiting between individual points of contactstrips.

It is the objective of the invention to provide a vari able resistor,especially a series resistor for a DC. series motor, which is of smallsize, inexpensive to manufacture and of high duty construction.

In a resistor of the type initially named, this objective is achieved inthat the windings consist of flat strips of chrome nickel steel oranother material possessing a high specific resistance. which are formedby etching or galvanic denudation of a suitable metal sheet or bygalvanic deposition, and which are connected by an insu lating layerwith a thermally conducting base unit. In the resistor according to theinvention, the resistance material is first laminated onto insulatingmaterial and then etched. A plate made in this way is then applied ontoa thermally conducting base unit. This unit may be constructed as asupporting unit, such as a housing, or the thermally conducting baseunit may be connected by means of suitable attachment devices to asupporting unit. The individual resistance strips are preferablyconnected in series, but they may also be connected in parallel. Aseries circuit has the advantage that when set to maximum resistancevalue. all the conducting strips are loaded. This results in a uniformtemperature loading and distribution. In a parallel circuit, theadvantage achieved is that etching errors do not have an additiveeffect.

One form of embodiment of the invention provides that the metal sheet islaminated by means of an adhesive onto the insulating layer, before theresistance strips are etched out. For this purpose, a thermoplasticsadhesive possessing high elongation at rupture is preferably used.

The chrome-nickel steel which is preferably used constitutes a suitablematerial for the resistance strips, since it possesses a relatively highspecific resistance and good mechanical strength properties. Accordingto one form of the invention, it is provided that the material of theresistance strips contains more than 17 percent chromium and 7 percentnickel. Such a steel is corrosion resistant. so that its resistance isnot subject to variations during its operational life.

Another form of embodiment of the invention provides that the insulatinglayer is an insulating mat of fine glass fabric. Such a fine glassfabric mat is preferably impregnated or coated with an adhesiveresistant to heat and to disruptive puncture. A fabric mat of this typepossesses the advantage that the adhesive coating is uniformlydistributed between the thermally conducting base unit and theresistance strips.

According to the invention, mica may also be chosen for the insulatinglayer. In the invention, a composite material of mica and polyamidefibres is to be preferred to mica alone for the insulating layer. Aninsulating layer constructed in this way possesses relatively gooddielectric strength.

As initially mentioned, the surface temperature and the temperaturedistribution in the resistor is of extreme importance for the fixing andthe location of fixing. It is for example important to protect adjacentparts, for example cables or components of thermoplastics, fromexcessive heat emission from the resistor. In this connection, theinvention provides that the heat conducting basic unit is a plate andthat the heat conducting plate possesses slits, through which the heatdistribution in the plate is determined. The mechanically strongest partof the resistance plate, consisting of resistance strips, insulatinglayer and heat removal plate, constitutes the heat conducting plate, sothat the heat removal plate either itself forms the supporting unit, orthe resistance plate is attached by means of the heat removal plate to asupporting unit. In addition, the heat removal plate has good heatconducting properties, so that intensive heat conduction can take place,for example towards the fixing. If therefore special precautions are nottaken, at least to reduce this thermal conductance, it will not bepossible to use thermoplastics or other materials of limited temperatureresistance for fixing. By means of slits in the heat removal plate. aparticular temperature flow can therefore be achieved. In thisconnection, the invention also provides that transverse slits areincorporated between attachment points of the heat removal plate to asupporting unit and the resistance strips, these slits reducing heatconductance to the attachment points. On the side towards the windingstrips, the attachment points are then sufficiently shielded againstheat, even if the resistance strips lie relatively close.

For many applications, narrow winding strips are necessary. This canlead to relatively high local temperature loadings, which in turn hasthe result of softening the thermoplastics adhesive. To counteract thisdrawback. a further form of embodiment of the invention provides thatthe resistance strips comprise increases in width at predeterminedpoints. In a further embodiment it is provided for this purpose that theincreases in width lie in the regions of smallest radius of curvature ofthe resistance strips. The widened portions are made by non-etching andappreciably increase the peeling resistance of the adhesive, especiallysince they constitute a reduced local resistance due to the increase inarea and therefore do not become so hot, as the remainder of theresistance strip.

In order that the resistor according to the invention can be operated asa variable resistor with a slider, contact strips connected to thewinding strips are necessary, it being possible to bring the slider intoengagement with these contact strips. In this connection, a fur therembodiment of the invention provides that the contact surfaces arelikewise flat strips of chromenickel steel, which are formed by etchingor galvanic removal of the metal sheet or by galvanic deposition andwhich are connected by means of the insulating layer to the thermallyconducting basic unit.

The contact strips are separated from one another by a so-called etchseam. The width of the seam determines the disruptive strength and thetendency to spark-drawing when the sliding contact of the slider movesover from one contact strip to another. One form of embodiment of theinvention therefore provides that the seam between resistance strips andcontact strips widens out upwards starting from the insulating layer. Inthe resistance strips, the smaller dimension of the seam at the base ofthe etch results in more effective adhesion of the resistance strips tothe insulating layer. In the contact strips, there is the additionaladvantage that the wear of the sliding contact is appreciably reduced,because the hard edge of the resistance material is appreciably softenedby the form of the etch seam.

In a further form of embodiment of the invention, it is provided thatthe thermally conducting basic unit possesses perforations in the regionof the contact strips. Where the thermally conducting basic unit is ofmetal, an appropriate recess in the basic unit beneath these largelyprevents short-circuiting to the heat conducting basic unit. Burningthrough of the insulating layer at this point therefore does not resultin notable damage.

The contact strips must be so arranged and shaped that, with regard totheir size and geometrical dimensions, the desired stepped adjustabilityof the resistor is ensured without excessive wear, without sparking orwithout excessive heating occurring. In this connection the inventionprovides that the slider possesses two sliding contacts spaced apartfrom each other and that the contact strips are arranged alongside eachother and so shaped that, when the slider is moved, only the one slidingcontact initially moves over onto the adjacent contact strip, while theother sliding contact still slides upon the previous contact strip. Thestaggered stepping of the contact strips permits a more favourablechangeover from one resistance step to the next, since sparking is to agreat extent eliminated.

Another form of embodiment for the contact strips provides that theslider possesses at least two sliding contacts spaced apart from oneanother, and that the contact strips are sub-divided into individual,mutually offset sections, so that in intermediate positions, the onesliding contact is in contact with the contact strip section of the nextresistance strip, while the other sliding contact still slides upon thecontact strip section of the preceding resistance strip.

In this form of embodiment, the resistance strip is connected to atleast two contact strips, the offset or staggered arrangement of thecontact strips once again appreciably reducing sparking.

Another form of embodiment for the construction of the contact stripsprovides that the individual contact strips are arranged concentricallyin arcs radially one after the other and their ends are mutuallystaggered circumferentially, and that the slider possesses one slidingcontact for each contact strip. The contact strips can be varied in anyway with regard to their contact width and can be individually suited tothe particular current intensities.

The arrangement and construction of'the resistor according to theinvention can be of any suitable type and the thermally conducting basicunit is a plate, which at the edge is at least partly raised and bentinwards, that the sliding contact of the slider is mounted upon acontact spring, by which the slider is pressed upwards with oppositeends against the inwardly bent edge of the section, which serves as aguide, and that the slider is pivotally mounted upon the resistanceplate, constituted of the resistance strip, insulating layer and heatremoval plate, Here, the resistance plate with the slider constitutesone unit, since the heat removal plate also acts as the guide elementfor the slider. With such a form of construction, relatively narrowtolerances are achieved.

In this connection the invention further provides that there is mountedupon the resistance plate a holder for a pivotal actuating lever, whichcan be brought into engagement with the slider arm by means ofa driver.The slider and actuating lever may be arranged and constructed in anysuitable manner. In this connection the invention further provides thatthe pivot axes of the actuating lever and slider are mutually offset.Such offsetting of the two pivotal axes results in a transmission rationot equal to unity, so that appropriate relationship can be achievedbetween the actuating range of the actuating lever and the sliding rangeof the slider.

As already mentioned above, the insulating layer supporting theresistance strips can be glued to the thermally conducting basic unit orheat removal plate, for example using a thermoplastics adhesive. Fornumerous applications, especially where the areas are large, glueing maybe accompanied by disadvantages, so that mechanical connection betweenthe insulating layer and the heat removing plate is more appropriate.The invention provides in this connection that projecting sheet noses orlugs are formed from the raised edge section of the heat removal plate,these lugs being bent in wards and pressing the plate consisting of theresistance strips and insulating layer against the heat removal plate.

The form and material of the slider can likewise be selected in anysuitable manner. One form of embodiment of the invention provides thatthe slider is formed from a strip of spring material. The strip is firstpunched out and then plastically deformed, for example to construct thecontact spring, so that it is ready for installation. For a slidersuitable for incorporation into a resistor. in which it is guided by theinwardly bent edge section of the heat removal plate, the inventionprovides that at opposite ends facing points are provided, which bearagainst the underside of the inwardly bent edge section, This provides aconducting path from the slider through the facing points to the heatremoval plate, from which the current can be conducted away by suitablemeans, for example a connecting lug.

In a further form of execution of the above-described slider, one formof embodiment provides that, at one end ofthe metal strip, a tongue ispunched out, likewise bent downwards by means of plastic deformation andcarries at its free end a bearing journal, which is seated in acorresponding recess of the resistance plate. The slider can be pivotedin one plane by means ofthisjournal, the sliding contact on the contactspring sliding over the contact strips, and the contact spring pressingthe slider via the facing points against the inwardly bent edge sectionof the heat removal plate.

. When the contact strips are staggered, the slider must possess two ormore sliding contacts. For this purpose the invention proposes that whenthere are two sliding contacts, a second contact arm is provided whichis constituted of a punching out from the first contact arm, that thesecond contact arm is bent downwards by plastic deformation and that thefirst, longer Contact arm is downwardly in the end region, thus makingit shorter. Such a form of construction permits free springing of thecontact arms, without mutual interference.

As already explained above, a slider constructed in this way is actuatedby the driver of an actuator. This must be capable of engaging in asuitable manner with the slider. For this purpose, one form ofembodiment of the invention provides that the metal strip possesses arecess, with which the driver of the actuating lever can engage. Thelatter may also favourably be constructed of a strip, a portion of whichis bent downwards to act as the driver.

As mentioned above, the thermally conducting base unit can also act asthe supporting unit, by means of which the resistor is mounted at itsfixing location. In this connection a particular form of embodimentprovides that the thermally conducting basic unit is also constructed assupporting unit for installation purposes and that the slider is alsoactuating lever, out of which a contact spring carrying a slidingcontact is formed. This results in one compact unit, distinguished byits especially small dimensions.

The supporting unit can be suited to the particular requirements andconstructed in any appropriate manner. For this purpose one form ofembodiment provides that the thermally conducting basic unit constructedas supporting unit is box-shaped and that the layer of resistance stripand insulating layer is secured to two or more sides of the basic unit.This enables an especially favourable heat distribution to be achievedfor the resistor. In a further form of embodiment for this purpose it isprovided that the highly loaded resistance strips are mounted upon thevertical sides of the basic unit. The vertical arrangement of the highlyloaded resistance strips causes a certain convection effect, whichpromotes cooling of the resistance strips by air flowing past them.

In another form of embodiment, the slider may possess more than twosliding contacts, so that a wider possibility of variation for theswitching point of the individual steps is afforded. It is thereforeproposed according to the invention that theslider is formed from ametal strip, possessing facing points at opposite ends, which bearagainst the underside of the inwardly bent edge section, that a numberof contact springs are provided extending transversely to thelongitudinal axis of the slider and carrying at their free ends slidingcontacts, these contact springs being formed as an integral part of aspring metal strip, which is held by the slider. Such a form ofconstruction of the slider is also suitable for resistors, in which theslider constitutes, together with the resistance plate, one unit.

The attachment of the spring metal strip to the metal strip can also becarried out in any suitable manner. The contact springs of the slidershould as far as possible exert an effective and uniform contactpressure. It is also desirable that the spring metal strip shall besimply mounted upon the strip. A further embodiment of the inventiontherefore provides that the spring metal strip is furnished with opposedprojecting sections, which seat in corresponding recesses of the slider,and

that the distance between these recesses is so selected that the springmetal strip is held under a prestress.

The spring metal strip, from which the contact springs are integrallyformed, may favourably be downwardly convex. It can then be removed outof the recesses, by bending it still further.

For numerous applications it is desirable for the supporting unit forthe resistor to be of plastics, especially of thermoplastics. Care musttherefore be taken that the heat evolution, which occurs when theresistor is in operation, cannot cause damage to the plastics. Theinvention provides for this purpose a form of construction which ischaracterised in that the resistance strip, insulating layer and heatconducting basic unit consitute a resistance plate, which is mounted ata distance from a supporting plate, that an actuating lever is pivotallymounted on the supporting plate, and that a contact spring carrying asliding contact is held by the actuating lever, this contact springbeing inserted selflockingly in a hole of the actuating lever. Here, thesupporting plate and actuating lever can be made from plastics, whileonly the contact spring consists of an electrically conducting, springmaterial. The spring is preferably constructed according to theinvention, in that it is joined integrally at its rear end to a contactlug, and that the contact spring is at least partially attenuated incross-section. A certain thickness is necessary in the contact spring onaccount of the contact lug but this has an effect upon the resilience.Therefore, reduction in cross section is provided, for example by havingone portion of smaller thickness. In order that the resistor slider canbe made as inexpensively as possible. it is advantageous for it to bemanufactured from simple spring steel. In such a case it is howevernecessary to construct a special sliding contact. For this purpose it isproposed by the invention that the contact spring shall possess at itsfree end a slit, which constitutes a web running transversely to thelongitudinal axis of the contact spring, upon which a contact element isheld as a rocker. The contact, rocker-mounted upon the contact spring.preferably consists of a material of high conductivity. such as bronze,brass or the like.

As already stated above. the holder for the actuating device of theresistor can be formed at the resistance plate constituted of theresistance strips, insulating layer and heat removal plate. Aparticularly preferred form of embodiment provides that the support forthe holder is by means of domes, bearing against the resistance strips,that the domes have an internally threaded hole serving to receive ascrew passing through a hole in the resistance plate and bearing withits head against the outside of the heat removal plate. Preferably, thisinternal hole is partially widened outwards leaving a gap between thescrew and the wall of the hole. Especially when an insulating washer isadditionally used together with the screw, a temperature drop isproduced inside the screw towards the thread. ensuring a firm fit of thescrew joint. If, as a result of some overload, the thermoplasticshousing material flows, complete loosening of the resistance plate doesnot result therefrom, since the air gap thus produced means that thedomes no longer bear and this gap itself acts as an insulator, so thatno further temperature flow takes place.

The resistance strips must be furnished with a connecting lug, or theymay be electrically connected to the heat removal plate. This can becarried out by various ways or means. One form of embodiment of theinvention proposes for this purpose that the resistance strips areconnected to the heat conducting plate and- /or to a connecting lug bymeans of a hollow rivet. The hollow rivet, when heated, causes anascending air flow. resulting in a reduction of temperature at theconnecting lug or heat removal plate.

Various examples of embodiment of the invention will be described inmore detail below with reference to the drawings.

FIG. 1 shows a section through a resistance place according to theinvention.

FIG. 2 shows a diagramatic plan of resistance strips of a resistanceplate.

FIG. 3 shows a resistance plate with attachment lugs.

FIG. 4 shows a plan upon contact strips of a resistance plate.

FIG. 5 shows a plan of another form of embodiment of contact strips.

FIG. 6 shows a further form of construction of contact strips.

FIG. 7 shows in perspective a resistance plate with slider.

FIG. 8 shows a section through the arrangement according to FIG. 7,together with a slider actuator.

FIG. 9 shows a section through the fixing of the resistance plateaccording to FIG. 7.

FIG. 10 shows a plan view of a slider for a resistance unit according toFIG. 8.

FIG. II shows a section through the slider according to FIG. 10.

FIG. 12 shows a side view of the slider according to FIG. 10 in contactwith a contact strip.

FIG. 13 shows a plan upon a slider for contact strips according to FIG.5.

FIG. 14 shows a section through the slider according to FIG. 13.

FIG. 15 shows a side view of the slider according to FIG. 13.

FIG. 16 shows a plan upon a slider for contact strips according to FIG.6.

FIG. 17 shows a section through a slider according to FIG. 16.

FIG. 18 shows a side view of the slider according to FIG. 16.

FIG. 19 shows a section through an attachment for a holder at aresistance plate.

FIG. 20 shows the attachment of a resistance lug.

FIG. 21 shows an electrically conducting connection between theresistance strip and the heat removal plate.

FIG. 22 show-s a section through another form of embodiment of aresistance unit.

FIG. 23 shows in enlarged section a resistance actuator of a resistoraccording to FIG. 22.

FIG. 24 shows in perspective a contact spring and a contact for anarrangement according to FIG. 23.

FIG. 25 shows a plan upon an installed contact.

FIG. 26 shows a partial plan upon an attachment lug for a resistanceplate.

FIG. 27 shows an exploded view of an attachment for the attachment lugaccording to FIG. 26.

FIG. 28 shows a further form of embodiment of the resistor according tothe invention.

FIG. 29 shows another form of embodiment of the resistor according tothe invention.

FIG. 1 is a section through a resistor according to the invention,without slider. A resistance strip 8 of chrome-nickel steel is connectedby means of an adhesive layer 9 to an insulating layer 10. A heatremoval plate 11, consisting of heat conducting material, is attached bymeans of a further ashesive layer 9a. The resistance strip has been madeby appropriate etching of a chromenickel steel sheet, which waspreviously glued to the insulating layer. The etching produces, betweenthe individual resistance strips, etch seams, one of which is shown at13 in FIG. 1. The etch seam l3 preferably is of smaller dimensions atthe bottom than at the top edge. The construction of the etch seamfacilitates maximum bearing surface for the resistance strip 8 upon theinsulating layer and thus optimal adhesion and heat removal. In theslider region, the shape of the seam 13 also has the advantage that thesliding contact wear remains small. Furthermore, sparking is avoided.The insulating layer may consist for example of a composite micamaterial having high temperature resistance. This is also true for aninsulating cover plate 12, which is applied onto the resistance strip.The adhesive layer 9a may be omitted, if the insulating layer is laidloosely upon the heat removal plate 11 and the parts are to be joinedtogether by mechanical fixing means.

The sandwich construction according to FIG. 1 is so arranged that thedimensions X and X are as small as possible. A small dimension X,facilitates good heat removal via the heat removal plate 11. Also, theselection of the dimension X, is substantially dependent upon thedielectric strength of the insulating layer 10. Below the etch seam 13,a perforation a is indicated, which will be discussed later in moredetail with reference to other figures.

FIG. 2 shows a diagramatic plan view upon a resistance plate accordingto FIG. 1. Three resistance strips are shown. 60, 61 and 62, the firstofwhich runs around the perimeter of the plate, while the innermoststrip 62 is partially meandering. The resistance strips 60 and 62 areconnected to contact strips 26 approximately in the middle of theresistance plate; the contact strips are not shown in more detail here.The resistance strips 60 and 62 can be connected in series or parallel.This also is not visible from FIG. 2. As can be seen, the strips are ofdifferent widths. By this construction, the temperature loading can beregulated in accordance with the power or rating. The innermostresistance strip 62 possesses circular widened portions 25, which areespecially advantageous when the resistance strips are very narrow andwhich ensure improved adhesion between the resistance strip and theunderlying insulating layer 10. Since the widened portions '25 result inreduced local temperature loading, this means that the thermoplasticsadhesive connecting the resistance strip to the insulating layer doesnot soften. Moreover, the widened portions 25, which are produced bynon-etching,

appreciably increase the peeling strength of the adheslon.

FIG. 3 shows a special construction of a heat removal plate 63, such asshown in section in FIG. 1. The layer 16a of resistance strips (notshown individually) and insulating layer is mounted upon the plane plate63. This may be done, for example, by lamination in an autoclave. In thevicinity of the slider region 26, which also is not shown in detail, aconnecting lug 64 is indicated. This connecting lug 64 constitutes oneterminal for the resistor, while the other is connected to the slider,not shown. The heat removal plate possesses, at three sides, fixing lugs22 with holes 24 for attaching to a supporting unit or the like.Circular arc slits 23 are formed on the side towards the resistancestrips around the attachment holes 24, screening the attachment holesagainst excessive heating.

FIG. 4 shows a form of embodiment of contact strips for a slider region26 (compare FIGS. 2 and 3). A slider illustrated diagrammatically at 1possesses two sliding contacts 3a and 3b at different radial distances.The sliding contacts 3a and 3b slide upon mutually staggered contactstrips 65 and 66 respectively.

As can be seen, two contact strips, staggered relative to each other,belong to each resistance strip. This means that the jump of the sliderfrom one resistance strip to the other is again sub-divided, thusreducing the sparking effect when transferring the contacts from onecontact strip to the next. The actuation of the slider 1 is limited atboth sides by two abutments 27a and 27b. Preferably, only the area ofthe slider range 26 bounded by the peripheral line 28 is open to engagement by the sliding contacts, while the remaining portion (not shown) ofthe resistor as shown in FIG. 1, is covered by an insulating cover plate12.

By suitable formation of the contact strips, they may be arranged tosuit the travel-resistance curve. For example the contact width may bevaried (g and f). The offset of contact strips 65 relative to contactstrips 66 can also be modified (dimension e). The contact strip lengthcan be varied (dimensions a and c) according to the current intensities.A measure of the spark-over resistance is the distance between adjacentand opposite contact strips (dimensions d and b). As already explainedabove, (FIG. 1), a special shape of the etch seam enables a high degreeof adhesion strength to be achieved, with a low tendency to sparking. Inorder to inhibit disruption towards the heat removal plate in the sliderregion 26, thechangeover regions between the individual contact strips65 and 66 may be punched out by appropriate perforations (see FIG. 1).This enables short-circuiting to be largely eliminated. Burning throughof the insulating layer, caused by breakdown sparking, does not resultin any notable damage.

FIG. 5 shows another form of construction of contact strips 66, overwhich a slider 67 moves. Five contact strips 66, at radial intervals,are provided, mutually offset circumferentially. The associated slider67 accordingly possesses five sliding contacts, which can be broughtinto engagement with the individual contact strips 66 successively.Regarding the dimensions a to the same applies as that stated relativeto FIG. 4. The dimensions d, to I1 represent the mutual offsetting ofthe individual contact strips, or the width of the etch seams.

In FIG. 6, a contact strip 68 is associated with each resistance strip.The contact strips 68 are of S-shape, so that a slider 69 possessingcircumferentially and radially offset sliding contacts 70 and 71 engagesan adjacent contact strip, when the slider 69 is pivoted. The contactstrips 68 are otherwise ranged in a circular arc, concentrically to thepivot point of the slider 69.

In FIG. 7, a plate 16, consisting of an insulating layer with resistancestrips glued thereon, is laid upon a metal plate 72, the edges 19 ofwhich are bent through to constitute stiffening. The plate 16 is securedby means of noses or lugs 18 formed from and bent from the edge sections19. The longitudinal edge sections also possess inwardly bent sections73 and 74, beneath which the ends of a slider la are inserted, thisslider bearing with contact springs on the slider region 75 of the plate16.

FIG. 8 shows the arrangement of the slider 1a more clearly. It possessestwo contact springs 76 and 77, which slide with sliding contacts uponthe sliding region 75. It also comprises a tongue 32, fitted at its endswith a pivot pin 33, which is mounted in an opening 80 in the plate 16.The contact springs 76 and 77 press the slider la against the inwardlyoriented edge sections 73 and 74, the slider Ia bearing with contactpoints 29 against the edge sections 73 and 74. The arrangement shown ofslider 1a and resistance plate with bent edge sections constitutes oneintegral unit, which ensures extremely small tolerances.

Domes or studs 17 are secured by screws 82 to the plate 16 and bearagainst a holder 15. The holder 15 journals, by means of a journal 83,an actuating lever 14, from which a downwardly bent driver 84 engages ina hole 30 of the slider 1a, so that when the actuating lever 14 ispivoted, the slider is also pivoted. The holder 15 possesses an opening86, through which the driver 84 extends downwards. The axes of thejournals 33 and 83 are mutually offset by a distance y. This offsettingdetermines the transmission ratio between the actuating lever 14 and theslider In.

FIG. 9 shows in more enlarged detail the attachment of the resistancestrips for the resistors according to FIGS. 7 and 8. The sandwichconstruction of the resistance plate 16 corresponds to that of FIG. 1.The same reference numerals have therefore been used. A nose or lug 18of strip is bent inwardly from the raised edge section 19, as seen inFIG. 9; this nose presses the resistance strips glued to the insulatinglayer 10, which are laid loosely upon the lower heat removal plate,against that plate.

In FIGS. 10 to 12, a slider is shown, such as can be used for theresistor according to FIGS. 7 and 8. The contact springs 76 and 77according to FIG. 8 are formed by arms 31a and 3112, which areconstructed by punching out and plastic deformation in the mannerillustrated in FIG. 11. The inner arm 31]) is cut out by suitablepunching and bent downwards by plastic deformation. It carries a slidingcontact 83. The outer, longer arm 31a surrounds the front region of thearm 31b and is bent downwards through 90 at 34 (FIG. 11). The result is,that without complete punching out, the arms 31a and 3111 can be movedindependently of one another. The arm 31a carries a sliding contact 83a.As can be seen also from FIGSv 10 to 12, the slider is shaped completewith slide arms 31a and 31b and the tongue 32 from a single flat stripof spring metal. At the four corners of the approximately rectangularstrip, are located contact points 29 u and 2912, by means of which theslider In (FIG. 8) slides on the under side of the edge sections 73 and74.

FIG. 12 shows the slider according to FIGS. 10 and II in the installedposition, the sliding contacts 83 and 83a sliding along the slide region75 comprising the individual contact strips. The arrows P to P,-,explain the force conditions at the resistor. The slider is soconstructed, and the contact points 29 are so arranged, that tiltingofthe slider when it is moved is as far as possible prevented. Thisdepends, among other things, upon the selected values of the dimensionsIt, and 11 and 1,.

FIGS. 13 to 15 show a slider which is suitable for contact strips of thetype shown in FIG. 5. This slider can also be incorporated in the sameresistor as illustrated in FIGS. 7 and 8. The slider according to FIGS.13 and 15 is once again made from one flat strip, which is provided withcontact points 29 at its four corners. The contact points 29 bear, inthis case also, against edge sections as described for example in FIGS.7 and 8. By contrast to FIGS. 10 to 12, the contact springs are hereprovided as a separate component. This is made from a flat spring metalstrip 85, from which contact arms 86 are integrally formed, holdingcontacts 87 at their ends. The contact springs 86 are plasticallydeformed out from the plane of the spring metal strip 85. The strip ofthe slider possesses, at each narrow end, downwardly bent edge sections88, furnished with openings 89, through which the projections 90 of thestrip penetrate. In this installed position, the strip 85 is downwardlycurved and thus prestressed as a spring, so that the contact springs 86are pressed downwards with a specific spring force. In other respects,the slider according to FIGS. 13 to 15 is constructed in the same manneras that according to FIGS. 10 to 12, and it bears the same references.

FIGS. 16 to 18 show a further form of embodiment of a slider, such ascan be used for contact strips of the type shown in FIG. 6. In otherrespects, the slider according to FIGS. 16 to 18 is similarlyconstructed to the slider previously described, so that the samereferences are used for the same parts. It can also be used for aresistor of the type illustrated in FIGS. 7 and 8. In this form ofconstruction, two spring arms and 91 point towards each other, the frontend regions lying alongside each other. A rectangular hole 93 serves toreceive the driver 84. By means of this slider, the radially andlaterally offset arrangement of the contacts 70 and 71, for the contactstrip layout of FIG. 6. is achieved.

It can be seen from FIG. 8 that the holder 15 for the resistor actuatoris supported by means of domes or studs 17 against the resistance plateand secured there by screws 82. A more detailed representation of thisattachment is given in FIG. 19. As can be seen, the dome 17 possesses aninternal hole 94, provided with a thread 43, which engages with thethread of the screw 82. In the lower part, the hole 94 is widened out,giving an annular gap 45 between the dome 17 and the screw 82. The headof the screw 82 bears against the lower heat removal plate of theresistor plate 16 through an insulating washer 46. The dome l7 bears atits lower end against the resistance strips or insulating layer of theresistance plate 16. The form of attachment illustrated achieves in asimple manner a temperature drop from the resistance plate 16 to theholder 15.

FIGS. 20 and 21 show a section through a resistance plate, correspondingto the section of FIG. 1. A stepped opening is formed through theresistance plate, and a hollow rivet 48 passed through it. In the regionof the heat removal plate 11, the opening is so broad that there is nocontact between the rivet 48 and the heat removal plate 11. By-means ofthis rivet 48, a connection lug 64 is attached, which bears through aninsulating washer 47 against the heat removal plate 11. With this formof the hollow rivet, heat generation produces an air flow whichcontributes to cooling the arrangement and improves the heatdistribution.

In FIG. 21, the resistance strip 8 is directly connected by means ofahollow rivet 96 to the heat removal plate 11.

FIG. 22 illustrates another form of embodiment for the construction of aresistor. A resistance plate 16, built up as shown in FIG. 1, issupported by insulating supports 97 from a supporting plate 99 ofplastics. The supporting plate 99 journals, by means of a pivot pin 100,an actuating arm 14a, into which a sliding contact spring lb with asliding contact 3a is inserted. The actuating lever 14a is also mouldedfrom plastics. The construction of the actuating lever and the contactspring can be clearly seen from FIGS. 23 to 25. The contact spring 1bconsists of a metal strip, at which a connecting lug 36 is formed at therear end. The connecting lug 36, of smaller width, constitutes anabutment 38, which bears against an abutment of the actuating lever 140.An upwardly bent locking finger 39 engages in a recess 101 in theactuating lever 14a. By these means, the spring 1b is heldself-lockingly in the actuating lever 140. Since a specific thickness isnecessary for the connecting lug 36, and also the contact spring 1b mustpossess determined spring properties, a region 37 of reducedcross-section is formed by the cut-out constituted from the lockingfinger 39; this region of reduced cross-section may also be of smallerthickness.

At the front end, the contact spring 1b possesses a rectangular slot102, which constitutes a web 40 perpendicular to the longitudinal axis.This web 40 and the slot 102 serve to receive the sliding contact 30.This contact is bent from highly conducting U-shaped metal strip, thetwo webs of which each contain a keyholeshaped recess 103. The width rat the narrowest part of the recess 103 is somewhat larger than thethickness s of the web 40. The width t of the web 40 is greater than thewidth r of the narrowest part and smaller than the width of the lowerportion of the recess 103. When assembling. the contact piece 3a isfirst rotated through 90 with the recess 103 slid over the web 40 (asshown in dotted line in FIG. 25). The contact piece 3a is then rotatedthrough 90. after which it becomes held as a rocker on the contactspring lb. The width of the contact piece 3a is so selected that it isguided as a neat fit in the slit 102. With this form of construction,accurate lateral guidance is ensured for the contact piece. This is ofextreme importance, since when the contact strips are finallysub-divided, tilting would lead to jumping over a contact point.

FIG. 26 shows a portion in plan of the arrangement according to FIG. 3,the insulating layer a and resistance strips 8a being also shown here.FIG. 27 shows a method of fixing this arrangement in a housing or onto asupport plate 104. For this purpose. the opening 24 possesses anaxially, downwardly bent flange 49, which fits into the internal holeofan insulating piece 50. The insulating piece 50 in turn fits with itsexternal diameter into the axial flange a of the supporting plate 104,

which likewise is provided with an opening 105. A hollow rivet 52 ispassed through the openings 105," the hole 50 and the opening 24 throughan insulating washer 53, in order to join the individual componentstogether. With this form of construction, the individual parts areaccurately centered. Also, unfavourable temperature influences upon thesupporting plate 104 are avoided, so that there is no need whatever forthis plate to be made of a highly thermally stable material. The hollowrivet 52 can, however, be omitted also; the resistance plate being thendirectly held in position, without further fixing means, solely by thecontact pressure of the slider.

FIG. 28 illustrates a form of embodiment, in which the element 106,constituted of resistance strips and insulating layer, is laminated upona supporting plate 20, or glued thereto or attached by means of othersuitable fixings, not shown. An actuating lever 107 is also providedwith a contact spring 108, sliding upon the sliding region of the plate106. The lever 107 is pivotally connected to the supporting plate 20 at109. As can be seen, the lever 107 can adopt three stop positions,formed by a bent up and perforated edge portion of the supporting plate20. A further lever 110, journalled in g the supporting plate 20, servesfor actuating another device, for example an air valve in an automobile.

In FIG. 29, a layer 111 consisting of resistance strips and insulatinglayer is applied onto a box-shaped supporting unit 21. The resistancestrips which can be highly loaded are provided on the vertical surfacesof the supporting unit 21. The resultant convection effect leads tobetter cooling of this arrangement. Here again, an actuating lever 112,provided with a contact spring 113, is pivotally mounted upon thesupporting unit 21.

The invention claimed is as follows:

1. A variable resistor comprising a thermally conductive base, anelectrically insulating layer, a resistance unit comprising a number ofresistance strips which have change-over spaces therebetween and whichare coupled by said electrically insulating layer to said thermallyconductive base, a slider having a contact which can be brought intocontact with contact surfaces of the individual resistance strips. andan actuating means for said slider wherein said base has open areas inalignment with said changeover spaces so that short circuiting throughsaid base due to electrical breakdown of said insulating layer issubstantially eliminated.

2. A variable resistor according to claim 1 in which said resistancestrips comprise a metal sheet secured onto said insulating layer.

3. A variable resistor according to claim 1 in which the material ofsaid resistance strips contains more than 17 percent chromium and 7percent nickel.

4. A variable resistor according to claim 1 in which said insulatinglayer is an insulating mat of fine glass fabric.

5. A variable resistor according to claim 1 in which said insulatinglayer comprises mica.

6. A variable resistor according to claim 1 in which said insulatinglayer comprises a composite material of mica and polyamide fibres.

7. A variable resistor according to claim 1 in which said thermallyconducting base is a plate and said thermally conducting plate comprisesslots which determine the heat distribution in said plate.

8. A variable resistor according to claim 1 in which said resistancestrips are covered with a nonconducting, non-metallic layer, the contactsurfaces, however, being left free.

9. A variable resistor according to claim 1 in which said contactsurfaces are flat strips of chrome nickel steel.

10. A variable resistor according to claim 1 comprising a contact springwherein said thermally conducting base is a heat removal plate, which atan edge thereof is bent at least partially upwards and inwards to form aguide and said slider is pressed upwards by said contact spring againstsaid inwardly bent edge section and is pivotally mounted on thecombination of said really mounted on said supporting plate, and acontact spring comprising a sliding contact is held by said actuatinglever, said contact spring being self-lockingly engaged in a recess ofsaid actuating lever.

13. A variable resistor according to claim 1 comprising a separate heatconducting means wherein said resistance strips are thermally coupled tosaid heat conducting means by a hollow rivet.

14. A variable resistor according to claim 1 wherein said resistancestrips are thermally coupled to said heat conducting means by a hollowrivet,

1. A variable resistor comprising a thermally conductive base, anelectrically insulating layer, a resistance unit comprising a number ofresistance strips which have change-over spaces therebetween and whichare coupled by said electrically insulating layer to said thermallyconductive base, a slider having a contact which can be brought intocontact with contact surfaces of the individual resistance strips, andan actuating means for said slider wherein said base has open areas inalignment with said changeover spaces so that short circuiting throughsaid base due to electrical breakdown of said insulating layer issubstantially eliminated.
 2. A variable resistor according to claim 1 inwhich said resistance strips comprise a metal sheet secured onto saidinsulating layer.
 3. A variable resistor according to claim 1 in whichthe material of said resistance strips contains more than 17 percentchromium and 7 percent nickel.
 4. A variable resistor according to claim1 in which said insulating layer is an insulating mat of fine glassfabric.
 5. A variable resistor according to claim 1 in which saidinsulating layer comprises mica.
 6. A variable resistor according toclaim 1 in which said insulating layer comprises a composite material ofmica and polyamide fibres.
 7. A variable resistor according to claim 1in which said thermally conducting base is a plate and said thermallyconducting plate comprises slots which determine the heat distributionin said plate.
 8. A variable resistor according to claim 1 in which saidresistance strips are covered with a non-conducting, non-metallic layer,the contact surfaces, however, being left free.
 9. A variable resistoraccording to claim 1 in which said contact surfaces are flat strips ofchrome nickel steel.
 10. A variable resistor according to claim 1comprising a contact spring wherein said thermally conducting base is aheat removal plate, which at an edge thereof is bent at least partiallyupwards and inwards to form a guide and said slider is pressed upwardsby said contact spring against said inwardly bent edge section and ispivotally mounted on the combination of said resistance strip andinsulating layer and said heat removal plate.
 11. A variable resistoraccording to claim 1 in which said thermally conducting base isconstructed to be a supporting unit for installation purposes and saidactuating means comprises a lever from which a contact spring carrying asliding contact is formed.
 12. A variable resistor according to claim 1in which said resistance strips, said insulating layer and saidthermally conducting base constitute a resistance plate, which ismounted at a distance from a supporting plate, said actuating meanscomprises a lever which is pivotally mounted on said supporting plate,and a contact spring comprising a sliding contact is held by saidactuating lever, said contact spring being self-lockingly engaged in arecess of said actuating lever.
 13. A variable resistor according toclaim 1 comprising a separate heat conducting means wherein saidresistance strips are thermally coupled to said heat conducting means bya hollow rivet.
 14. A variable resistor according to claim 1 whereinsaid resistance strips are thermally coupled to said heat conductingmeans by a hollow rivet.